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The Development of Tasmanian Shore Platforms

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The Development of Tasmanian Shore Platforms SECTION III FACTORS IN Tlffi DEVELOPMENT OF TASMANIAN SHORE PLATFORMS \ 273 INTRODUCTION Tasmanian shore platforms have been produced by a number of interrelated passive and active factors. Passive factors involve the attributes of the rock itself. Active factors include weathering, hydraulic action and environmental considerations such as climate, tidal regime and wave characteristics. Various aspects of shore platform development have been mentioned on previous pages in connection with specific platforms. In the following section the factors will be discussed system­ atically, drawing on field evidence, experimentation and the literature for background. PART ONE PASSIVE FACTORS 274 Chapter 13 BEDROCK CHARACTERISTICS Rock characteristics are very important to the potential development of a shore platform. If the rock type is not suitable, even the most effective forces will have difficulty in establishing horizontality. Three attributes of the rock are significant: (1) Structure, (2) Texture and (3) Chemical composition. Structure here refers to jointing and bedding patterns, texture is concerned with the characteristics and arrangement of the component rock particles, and chemical corn• position includes the chemical make-up of both rock particles and cement. STRUCTURE In general, well developed high tide shore platforms do not occur in Tasmania when horizontal jointing or bedding is absent or widely spaced. Two rock types, granite and dolerite, commonly show these characteristics. Granite has widely spaced, curving joint planes and dolerite is often well jointed vertically but lacking in uniform horizontal jointing. It is probable that weathering alone is not effective enough to bevel a surface without some previous help by hydraulic quarrying to a zone of weakness in the rock, As was shown at Grant Point, granite is not very susceptible to peritidal weathering, .and, for this 275 reason it could be argued that it is not so much the structure as the weathering properties which prevent the formation of well-developed platforms, However, dolerite weathers more rapidly in the peritidal environment and it too exhibits little horizontality, indicating that the jointing pattern may be the controlling consideration., The difference between the South Croppies Point and Don Heads platforms also suggests the importance of rock structure. South Croppies Point Basalts contain no consistent horizontal planes of weakness over much of their area and support no high tide platforms, Don Head13, on the other hand, is also composed of basalt, but the rock is well jointed both vertically and horizontally. In marked con- trast to the very poorly developed South Croppies Point plat­ form the Don Headsfeature is the best developed horizontal shore platform included in this study. Further evidence for the importance of structure is the similarity of morphology between platforms cut in differing materials, but having closely spaced joint and bedding planes. Platforms in sandstones and basalts often show similar morphologies, and approximately the same relative height compared to mean higher high water, In certain cases, the structure may even encourage a plat- form to develop independently of weathering. When a thick, massive and resistant bed is overlain by less resistant material, extensive platforms may result. An example occurs on the 276 cliffed coast seaward of the "blowhole", south of Eaglehawk Neck. A thick, poorly jointed bed of sandstone forms a plat- form at about twelve feet above mean higher high water at its highest point (Photo 31), The surface drops away along the coast to north and south, following a slight bowing of the formation. The beds above the resistant sandstone are thin and closely jointed - characteristics which promote effective removal. TEXTURE While the structure of the rock is probably of basic impor& ance in determining the presence or absence of a high tide plat­ fonn, the texture is significant in controlling the efficiency of the platform bevelling process, Bevelling, which puts the finishing touches on a surface already roughly planed by hydraulic action, is largely accomplished by salt crystallization. This process will be treated in detail in Chapter 16, In order for salt crystallization to be effective, the texture of the rock must allow salt water to enter for subsequent evaporation and crystal growth. Salt crystallization will be slow or non- existent when rock texture is unfavorable for water entry, resulting in poorly developed horizontality, The importance of the ability to take on water is demon­ strated by the results of tests conducted on rock samples taken 277 Photograph 31 Supratidal Structural Platform. The platform is located three miles southeast of Eaglehawk Neck. The resistant sandstone bed is highest above sea level at the point shown in the photo and drops in elevation to north and south. Photograph 32 High Tidal Structural Platform. The platform is located about 1000 feet north of the platform pictured in Photo 31. The high tidal structural platform is basically the top surface of the resistant sandstone bed of Photo 31. The bed occupies a lower level in Photo 32 because of gentle bowing of the fonnatibn. The increased dip brought the upper edge of the bed into the altitudinal zone where water-layer weathering is most effective. Weathering is now actively modifying a surface which was created largely by hydraulic action on differentially resistant beds. 278 from various coastal sites, Granite, dolerite, basalt and sandstone yielded data which indicated that the platforms which possessed greatest horizontality were carved in rocks which were most efficient at water intake (basalt and sand .. stone). Conversely, field evidence shows that even limited bevelling is rare in granite, the rock type least able to absorb water. A more complete discussion of the tests and conclusions drawn from them follows in the chapter on weather­ ing, where the importance of rock texture is repeatedly apparent, CHEMICAL COMPOSITION The third important passive factor in shore platform development is the chemical composition of the rock, Chemical composition is significant in two ways: (1) If the rock is calcareous the platform elevation will be distinctly lower than in the case with non-calcareous rocks. (2) If the rock is non- calcareous the chemical composition determines the relative efficiency of the various chemical forces acting upon it. Revelle and Emery (1957), Kaye (1959) and others have described horizontal platforms in aeolianite, beach rock and exposed coral rock which are located at or below the level of low tide, The general consensus is that these platforms are formed by solution, either by fresh water floating on the denser sea water, bio­ chemical reactions or solution by aereated sea water. 279 Unfortunately, the scarcity of limestone coasts in Tasmania has now allowed these observations to be verified in terms of the local environment. Limestones are found on the coast of Tasmania in few places, notably Marawah in the northwest and Pt. Hibbs in the southwest. The Marawah formation is an intertidal platform which is obviously being modified by sand scour and thus can not furnish much information on solution processes or elevations of horizontal platforms. The Pt. Hibbs platform is said to be very well developed, but its relatively inaccessible location makes field study difficult. When questioned, geologists who had visited the area stated that it was their impression that the platform surface was deeply awash at high tide. The Pt. Hibbs problem will have to remain unresolved until precise measurements can be made. Most of the rocks comprising the Tasmanian coast are non- calcareous, either in cementation or composition. Where these rocks show horizontality, the elevation is at or near the level of mean higher high water. The higher elevation of the non- calcareous platforms is probably due to the characteristics of the major bevelling mechanism, salt crystallization, which is limited by the saturation level. Chemical composition of the non-calcareous rocks is not as important as texture in the process of bevelling by salt \ 280 crystallization, but can be a contributing factor. Mineral breakdown due to chemical activity may prepare the rock for more effective subsequent salt crystallization, The chemical comp­ osition of the constituent minerals is closely related to the type of reactions which will be most effective in breaking down the rock, For instance, orthoclase, a potassium aluminum silicate, is very susceptible to hydrolysis,while olivine, a magnesium iron silicate, is often involved in oxidation re­ actions. Passive factors, inherent in the bedrock, exert a major control over the presence or absence of platforms on any rocky coast. In addition they are important in determining the processes which will be most effective in platform development. PART TWO ACTIVE FACTORS 281 Chapter 14 THE ENVIRONl-:!ENT The environment exerts a tremendous influence over shore platform development, Many environmental components affect the growing platforms, but climate, wave characteristics and tidal regime are probably most important. These three aspects of the environment furnish broad outlines for the types of form- ational activities possible in a specific location and also control the rate at which the processes operate. CLIMATE Temperature Climate, including temperature, precipitation and wind, is one of the most basic of the environmental considerations. Temperature exerts considerable control
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